The present invention relates to a method for determining the position of the focus of an opto-electronic apparatus, in particular of a bar code reader. Furthermore the invention is directed to an opto-electronic apparatus, in particular for carrying out a method of this kind.
In order to be able to use opto-electronic apparatuses of this kind, in particular bar code readers, as flexibly as possible, it is known to select the position of the focus, i.e. the distance between the point of convergence of the transmitted light beam and a fixed point determined with respect to the opto-electronic apparatus, for example the outlet opening for the light beam from the housing of the opto-electronic apparatus, from a series of positions of the focus predefined by the design of the apparatus. For this purpose the focusing lens can for example be displaced in the direction of its optical axis, or tilted, via a stepping motor, so that the distance between the lens and the element which transmits the light, for example a laser diode, is adjustable.
In the context of the manufacturing process of these opto-electronic apparatuses, stepping motor positions and lens positions are associated with one another in a calibration process, by means of which the desired values can be achieved for the different positions of the focus.
Depending on the application, very high demands exist with respect to the long-term stability of these values which are to be set for the positions of the focus, so that the tolerances for the individual components must be kept very small. By way of example, the required positional accuracy for the object distance (spacing laser diode to lens) can be less than .+-.0.5 .mu.m in an individual case.
As a result of these extreme requirements on the long-term stability of the geometrical arrangement of the individual optical elements, it is only possible to use methods for the adjustment of the lens which ensure the required high precision over the total expected working life of the opto-electronic apparatus. This method and the mechanical devices which are required for it are very complicated and thus very costly to manufacture.